The Great Pyramid: the precision, and a date that will not sit still

The tallest thing people built for about 3,800 years, set out so true that surveyors today find its sides a fraction of a degree off the compass, and still holding spaces no one has entered.

Set out true — MEASURED

Resurveyed in 2015 from 84 points where its original edge survives: each side about 230.4 metres, 756 feet, the longest and the shortest within a few inches of each other, and every side running within about a twelfth of a degree of true north, south, east and west, a fifteenth on average. The builders measured distance even better than angle: the base is a very slight parallelogram.

The height — CALCULATION

About 146.6 metres, 481 feet, when its casing was whole, and the tallest structure on earth for some 3,800 years.

Spaces no one has entered — MEASURED

In 2017 physicists reading the cosmic-ray muons that pass through the stone found a void at least 30 metres long above the Grand Gallery. In 2023 they mapped a sealed corridor behind the north face, about 9 metres long and 2 metres square, and a camera pushed through a joint in the stones showed it. What either was built for is not known.

A date that will not sit still — MEASURED · CONTESTED

The accepted date is Khufu's reign, about 2560 BC, from the king lists and from the work gangs' painted marks naming Khufu in the sealed chambers above the King's Chamber. Radiocarbon disagrees. Organic material from the mortar of the pyramids from Djoser to Unas, dated in 1984, came out on average 374 years older than their kings' dates; a larger set in 1995 came out 100 to 200 years older, and Khufu's own samples scatter across about 400 years. The usual answer is that the builders burned wood that was already old.

The man who shipped the stone — THE RECORD

In 2013 the oldest inscribed papyri yet found turned up at Wadi al-Jarf on the Red Sea: the logbook of an inspector named Merer, late in Khufu's reign, recording his crew's boat trips hauling white limestone from the Tura quarries across the Nile to Giza, two or three round trips every ten days. It is an eyewitness account of the building, and one of several logbooks in the same find.

How it was checked

• StatusMeasured: the survey, the scans, the radiocarbon · the record: the logbook · contested: why the dates run old · one real number, wrongly read
• SourcesDash and Paulson, The 2015 survey of the base of the Great Pyramid, Journal of Egyptian Archaeology 102 (2016) (https://journals.sagepub.com/doi/10.1177/030751331610200114); Archaeology, The Great Parallelogram (September/October 2016) (https://www.archaeology.org/issues/227-1609/trenches/4738-trenches-egypt-pyramid-measurement); Morishima et al., Discovery of a big void in Khufu's Pyramid by observation of cosmic-ray muons, Nature 552 (2017) (https://arxiv.org/abs/1711.01576); Procureur et al., Precise characterization of a corridor-shaped structure in Khufu's Pyramid, Nature Communications (2 March 2023) (https://www.nature.com/articles/s41467-023-36351-0); AERA, How old are the pyramids? (the 1984 and 1995 radiocarbon projects) (https://aeraweb.org/projects/how-old-are-the-pyramids/); Bonani et al., Radiocarbon dates of Old and Middle Kingdom monuments in Egypt, Radiocarbon 43 (2001) (https://www.cambridge.org/core/journals/radiocarbon/article/radiocarbon-dates-of-old-and-middle-kingdom-monuments-in-egypt/A967302ADD527BFEB9226457682C0B4A); National Geographic, the Red Sea scrolls: Merer's logbook (Tallet) (https://www.nationalgeographic.com/premium/article/red-sea-scrolls-pyramids-egypt); The height and the 3,800 years: the Great Pyramid of Giza (https://en.wikipedia.org/wiki/Great_Pyramid_of_Giza); Full Fact, The Great Pyramid's location isn't as spooky as this post makes out (https://fullfact.org/online/great-pyramid-speed-of-light/)